The present invention relates to electromagnetic wave reception and transmission apparatus and more particularly, to a reflector type antenna having elements disposed on the reflective surface thereof for suppressing unwanted sidelobes in the original antenna pattern.
Various techniques have been used in the past for nulling or suppressing selected sidelobes in directional antenna systems. One such technique is adaptive nulling, particularly suitable for phased array systems, where a null is electronically placed in the antenna response pattern to reduce the amount of energy received from a jamming signal source. Adaptive nulling, however, requires costly components, complex algorithms, often requires long signal processing times, and is not suitable for reflector type antennas.
A known technique for reducing sidelobe interference in reflector type antennas involves the combining of the signal from the main antenna with that of an auxiliary antenna, suitably adjusted in amplitude and phase. Such auxiliary antenna systems, however, are generally expensive to fabricate and require high precision components and complex microwave circuitry to achieve adequate system performance.
In U.S. Patent No. 4,376,940 issued to H. Miedema on Mar. 15, 1983, there is disclosed an antenna arrangement for suppressing sidelobes, which consists of a pair of auxiliary antennas located at the edges of the reflector antenna such that, when the energy from the main and auxiliary antennas are combined in a hybrid coupler and associated circuitry, certain sidelobes are suppressed. An alternate embodiment of the invention involves laterally offsetting edge segments of the reflector to produce a radiation pattern which is phased opposite to the selected sidelobes to be suppressed.
A problem associated with sidelobe nulling by moving the edges of the reflector or by widely-spaced auxiliary antennas is the resultant formation of grating lobes which manifest themselves in the antenna pattern. Moreover, the amount of nulling that can be achieved is limited, since only the electromagnetic energy that meets the edge of the antenna is effected. In most instances, the energy reflected from the vicinity of the edge or rim of the reflector to the feed horn is purposely made much lower than the energy from the center region of the reflector to the feed horn. This is done in order to reduce overall sidelobes.
The use of such auxiliary antennas for nulling is generally restricted to the receiving mode due to the added requirements imposed on the signal processing equipment when high transmitted power levels are involved. It will also be apparent that the retrofitting of existing antennas to include auxiliary antennas or adjustable edge reflector elements could be prohibitively expensive.